Conventionally, resolvers are used for detecting a rotation angle of a motor or the like. Such a resolver includes an excitation coil and detection coils. When an alternate current is supplied to the excitation coil, an AC voltage is generated at each detection coil in accordance with a relative angle between a stator and a rotor. These AC voltages are detected by voltmeters which are connected to the respective detection coils, and are then outputted to a resolver-digital converter (RD converter). The RD converter converts detection angles contained in respective resolver signals inputted thereto, into digital angle data, and outputs the digital angle data. In this manner, the resolver is used in combination with the RD converter.
Usually, a resolver is placed near a motor. Therefore, the resolver is significantly affected by a magnetic field generated by the motor, and noise that is caused by an influence of the magnetic field generated by the motor is superimposed on a resolver signal. While the motor is rotating, the magnetic field generated by the motor synchronizes with the rotation. Accordingly, the noise, which is superimposed on the resolver signal due to the magnetic field generated by the motor, is low frequency noise. In recent years, motors are often driven by PWM. (Pulse Width Modulation) controlled voltage. A resolver included in such a PWM-driven motor is affected by switching noise. There are cases where such switching noise is distributed not only in a low-frequency region but also in a high-frequency region. If such disturbance noise is superimposed on a resolver signal that is inputted to the RD converter, then the operation of the RD converter is affected by the noise. Accordingly, digital angle outputs from the RD converter contain angle errors caused by the disturbance noise.
In this respect, Patent Literature 1 proposes an RD converter capable of removing noise that is caused by an influence of a magnetic field generated by a motor. The RD converter includes: a synchronous detection circuit configured to perform synchronous detection in an angle calculation loop by referring to an excitation signal; a controller configured to control a digital angle output such that an output from the synchronous detection circuit becomes zero; and a band-stop filter, disposed between the synchronous detection circuit and the controller, configured to remove low-frequency noise components superimposed on a resolver signal. Here, a bandwidth of 2 kHz or higher, having an excitation frequency of 10 kHz as a center frequency, is set for the band-stop filter, and the band-stop filter is configured to remove frequencies within this bandwidth.